FIG. 5 shows a general block diagram of a conventional barcode reader using a linear image sensor. For example, in a linear image sensor made by Toshiba Corporation (TCD1103GFG), a CPU 10 generates an LED (Light Emitting Diode) control signal and controls light emission intensity of an LED 20 to illuminate a barcode.
Further, the CPU 10 outputs a master clock MCLK, an integration clear gate ICG and a shift gate SH to a linear image sensor 30 so as to control operation of the linear image sensor 30. A barcode image formed with an image pickup lens on a light receiving surface of the sensor is converted into an electric signal with the linear image sensor 30, and is transferred, as an output signal OS from the linear image sensor 30, to the CPU 10 side. The integration clear gate ICG is used to control frame rate of the linear image sensor 30, and the shift gate SH is used to determine whether or not an electronic shutter is to be operated based on clock pattern thereof.
FIG. 6 shows a representative example of control timing chart corresponding to the block diagram of FIG. 5.
In the conventional barcode reader, output frame time is determined based on the integration clear gate ICG of the linear image sensor 30. The shift gate SH of the linear image sensor 30 is set in an electronic shutter mode. Then exposure time of the linear image sensor 30 is determined by dividing the frame time as shown in FIG. 6.
Further, the data output time of the data output OS of the linear image sensor 30 starts from the rising edge of the integration clear gate ICG, and required time is determined based on a frequency f of the master clock MCLK of the linear image sensor 30 and the number of pixels N of the image sensor, N×2/f. For example, to output N=1500 pixels at the frequency of the master clock MCLK, f=1 MHz (mega Hertz), 1500×2/1 MHz=3 ms (milli-second) data output time is necessary. Accordingly, the frame time should be longer than 3 ms.
Further, regarding the lighting timing of the LED 20, the barcode reader turns on the LED 20 at the same time when a user starts the barcode reader, and continues the lighting of the LED 20 until the barcode has been read or a predetermined period of time has elapsed. Accordingly, as shown in FIG. 6, the LED 20 is continuously lighted for one frame time of the linear image sensor 30.